Reaction of 4‐chloro‐2‐methyl‐5‐(1‐methylhydrazino)‐3(2H)‐pyridazinone (1) with carbon disulfide followed by alkylation yielded 2‐alkylthio‐4H‐pyridazino[4,5‐e][1,3,4]thiadiazine derivatives 2. Oxidative cyclization of 5‐(4‐substituted 1‐methylthiosemicarbazido)‐3(2H)‐pyridazinone derivatives 4 with N‐bromosuccinimide also gave 2‐substituted amino‐4H‐pyridazino[4,5‐e][1,3,4]thiadiazine derivatives 5. Heating of 2 and 5 resulted in ring contraction to afford the corresponding pyrazolo[3,4‐d]pyridazine derivatives 6, 7 via sulfur extrusion. A possible mechanism for the desulfurization reaction is discussed, comparing with a structural difference between a type of pyridazino[4,5‐e][1,3,4]thiadiazine (2,5) and another one (9,11,13,15).